This invention relates to a semi-permeable hollow fiber gas separation membrane which possesses a non-external discriminating region. The invention further relates to methods for making and using such membranes.
In various industries, it is necessary or highly desirable to separate one gaseous component from other gaseous components in a gas mixture. Processes used to perform such separations include cryogenics, pressure swing adsorption, and membrane separations.
Membranes have been used to recover or isolate a variety of gases, including hydrogen, helium, oxygen, nitrogen, carbon monoxide, carbon dioxide, water vapor, hydrogen sulfide, ammonia, and/or light hydrocarbons. Applications of particular interest include the separation of air into enriched oxygen streams, which are useful for increasing the efficiency of fermentation processes and for enhancing combustion processes, and enriched nitrogen streams, which are useful for inert padding of flammable fluids and for increasing food storage times. Other applications of interest include the separation of hydrogen from gas mixtures containing gases such as nitrogen, carbon monoxide, carbon dioxide, and/or light hydrocarbons in addition to hydrogen. For example, the separation and use of hydrogen is often necessary in various hydrocracker, hydrotreater, and catalytic cracking processes used in the oil refinery industry. Membranes can be used to achieve such separations.
Such membrane separations are based on the relative permeability of two or more gaseous components through the membrane. To separate a gas mixture into two portions, one richer and one leaner in at least one gaseous component, the mixture is brought into contact with one side of a semi-permeable membrane through which at least one of the gaseous components selectively permeates. A gaseous component which selectively permeates through the membrane passes through the membrane more rapidly than at least one other gaseous component of the gas mixture. The gas mixture is thereby separated into a stream which is enriched in the selectively permeating gaseous component or components and a stream which is depleted in the selectively permeating gaseous component or components. The stream which is depleted in the selectively permeating gaseous component or components is enriched in the relatively non-permeating gaseous component or components. A relatively non-permeating gaseous component permeates more slowly through the membrane than at least one other gaseous component of the gas mixture. An appropriate membrane material is chosen for the gas mixture so that some degree of separation of the gas mixture can be achieved.
Membranes for gas separation have been fabricated from a wide variety of polymeric materials, including cellulose esters, polyimides, polyaramides, and polysulfones. An ideal gas separation membrane is characterized by the ability to operate under high temperature and/or pressure while possessing a high gas separation factor (selectivity) and high gas permeability. The problem is finding membrane materials which possess all the desired characteristics. Polymers possessing high gas separation factors generally have low gas permeabilities, while those polymers possessing high gas permeabilities generally have low gas separation factors. In the past, a choice between a high separation factor and a high gas permeability has been unavoidably necessary. Furthermore, some of the membrane materials previously used suffer from the disadvantage of poor performance under high operating temperatures and pressures.
Hollow fiber membranes are a preferred membrane configuration for gas separation applications because of their high surface area/volume ratio. The hollow fiber gas separation membranes of the prior art generally possess an external discriminating layer and an internal porous substructure, wherein the external discriminating layer and the internal porous substructure are comprised of the same polymeric material. A plurality of hollow fiber membranes are typically fabricated into a bundle and the feed gas mixture is introduced on the outside (shellside) of the hollow fiber bundle. However, introducing the feed gas mixture on the shellside of the hollow fiber bundle generally results in poor shellside flow distribution due to channeling and concentration polarization which adversely affect membrane separation performance. Internal (boreside) feed greatly reduces the problems associated with channeling and concentration polarization but boreside feed is not practical with conventional hollow fiber membranes possessing an external discriminating layer and internal porous substructure due to problems associated with condensation of the condensible components in the feed gas mixture within the porous substructure of the membrane, which adversely affects membrane separation performance. An additional problem encountered with conventional hollow fiber membranes possessing an external discriminating layer is damage to the external discriminating layer due to handling during fabrication and use which adversely affects membrane gas separation performance.
A membrane capable of separating a gaseous component from other gaseous components in a gas mixture which possesses high selectivity, high gas permeability, and ability to operate under extreme conditions of temperature and pressure is needed. Furthermore, what is also needed is a gas separation membrane possessing a non-external discriminating region so that boreside feed becomes practical and efficient and the membrane is less subject to damage upon handling during fabrication and use.